1. Field of the Invention
The present invention is directed in general to a method for avoiding over-convolutions in the phase coding direction in magnetic resonance tomography employing surface coils. In particular, the present invention is directed to a method of avoiding over-convolutions by adaptation of the parameters of a magnetic resonance examination to be undertaken, so that such over-convolutions do not occur or occur only to an innocuous extent.
2. Description of the Prior Art
Magnetic resonance tomography is a tomogram method for medical diagnostics that makes it possible to show structures of the human body to a significant extent and in detail. In principle, magnetic resonance tomography is based on the application of a strong external magnetic field in a region of a subject to be examined, causing a certain portion of the magnetic spins of the nuclei to align. More precisely, a statistically defined number of nuclear spins assumes specific energy levels. When a radio-frequency pulse supplies additional energy at a resonant frequency that is substance-specific and is dependent on the gyromagnetic constant, a specific number of nuclear spins assumes different energy levels. After the radio-frequency pulse is deactivated, the return onto the original energy levels can be measured in the external magnetic field by receiving the energy emitted at the resonant frequency.
In practical application, it is usually the distribution of the hydrogen in the human body that is measured, since, a good presentation of the tissue is established merely by detecting the hydrogen distribution due to its widespread nature within the human body in all tissues, and hydrogen can be detected especially well on the basis of its high gyromagnetic constant.
It is necessary for magnetic resonance tomography that the signal of a nuclear spin, referred to below as an MR signal, be able to be assigned to location information. A number of detailed methods are known for this purpose, including the use of combinations of additionally activated magnetic fields, excitation frequencies and readout times at the resonant frequencies. Fundamentally, the physical effect is always utilized that the resonant frequency of an MR signal in an external magnetic field is location-dependent when this external magnetic field no longer has a constant field strength, but changes over a distance, as is the case, for example, given an additionally applied, linearly rising or dropping magnetic field. Such a magnetic field is referred to as gradient magnetic field. Further, such a different field also causes various nuclear spins that initially resonate with the same phase position to diverge in phase after a certain time, and to retain this phase difference when the additional magnetic field is deactivated after a certain time and they again resonate with the same resonant frequency. This is the basic principle of phase coding.
A radio-frequency pulse at the resonance frequency of the nuclear spins in the basic magnetic field first excites the nuclear spins, and the overall magnetic moment of the nuclear spins is partially and entirely rotated (is referred to as 90xc2x0 excitation) into the plane perpendicular to the basic magnetic field. The magnetization vectors distributed in the measurement volume thereby essentially exhibit an identical phase after the excitation. Subsequently, two magnetic gradient fields are activated in two evaluation directions, referred to as the phase coding gradient and the frequency coding gradient. These magnetic field gradients have a linear course. Consequently, the resonant frequency and phase of the individual magnetization vectors are dependent on the location. The magnetic field is activated for a fixed, defined time t. When the phase coding gradient is in turn deactivated, then the phases of the nuclear spins have become different from each other in location-dependent fashion. This setting of the phases is preserved until the readout time. When the radio frequency resonance signal that the nuclear spins emit when they drop back from their excitation level into the basic level is received, then the phase position of the individual nuclear spins can be interpreted by mathematical methods, for example Fourier methods, and a location in the phase coding direction can be allocated to the phase. In phase coding, this switching is multiply implementedxe2x80x94for example, 256 timesxe2x80x94with different amplitudes. A different phase coding gradient having a higher value is thereby added at every repetition of the above-described method. The individual steps of the phase coding gradient usually exhibit an equidistant spacing.
The above-described, additional magnetic fields or gradient fields are generated by two coils that each generate a magnetic field, these fields being oppositely directed. A magnetic field arises in the volume element between the two coils that increases or decreases linearly over distance. The nuclear spins are excited by a radio-frequency excitation coils and the MR signal is received via the same coils or specific reception coils. Such specific reception coils are small, additional coils that, differing from the gradient evaluation coils, the basic magnetic field coil and the large radio frequency transmission/reception coil, are permanently installed in the nuclear magnetic resonance tomography apparatus. The reception coils are relatively small and compact and usually are flat and can be placed in the proximity of the organ to be examined. These specific reception coils are also referred to as surface coils.
In an examination with surface coils or with the large transmission/reception coil, the problem known as over-convolution can occur in the phase coding direction. It can occur in a phase coding method that a nuclear spin cannot be unambiguously topically assigned because a nuclear spin close to the middle of the coils in the region of a small gradient exhibits the same phase as a nuclear spin outside a selected field of view (FOV) in the region of a high gradient that has additionally rotated at least one full period of 2xcfx80. This case fundamentally occurs when the FOV is smaller in the phase coding direction than the subject under examination. Dependent on the number and size of the selected surface coils, these receive signals not only from the FOV but also from sections of the examination subject that lie farther away. Since a gradient field can still be noticed in these regions lying outside the field of view and a reception by the surface coil from these regions cannot be completely prevented, it can occur that the signals from this region exhibit the same phase as the signals from the actual field of view, and are no longer negligibly small, so that disturbances in the form of superimpositions thus occur. This particularly occurs given the employment of surface coils that are intended to represent only a small region in great detail and where large regions of the body of the person to be examined lie outside the actual field of view.
It is an object of the present invention to provide a method and an apparatus which avoid disturbing over-convolutions in magnetic resonance tomography, particularly MR imaging with surface coils.
This object is inventively achieved in a method according to the invention for avoiding over convolutions in the phase coding direction in magnetic resonance tomography, wherein at least orthogonal slices are measured, or a three-dimensional volume is measured, as an overview presentation in a first step. Slices for the following series of measurements then can be defined in the overview presentation, which are also referred to as localizers. In a further step, a calculation is made to identify overlapping phases and the amount of the appertaining signal is defined, this being a noise signal. If an over-convoluted signal theoretically derives from these calculations that exceeds a reference value, a warning is emitted and a selection possibility is offered in order to reduce the phase coding step width and to thereby enlarge the correctly analyzed field of view. When the offered selection is accepted by an operator, then an automatic reduction of the phase coding step width ensues in conjunction with a reduction of the spatial resolution in phase coding direction.
Advantageously, the above-described method also makes it possible for a rather an inexperienced operator to avoid such over-convolutions.
In an embodiment, the magnetic resonance tomography exposure ensues using a surface coil.
In a specific examination situation wherein a surface coil is employed, it is considerably easier for the operator of a magnetic resonance tomography apparatus to avoid over-convolutions. Since a surface coil can also receive signals from regions beyond the actual field of view that represent an over-convoluted signal relative to signals in the field of view, over-convolutions occur that are very difficult for inexperienced personnel to handle. Due to the automatic calculation and estimate of the potentially arising over-convolutions and the automatically ensuing indication thereof connected with a corresponding variation of the parameters, the operation of the magnetic resonance tomography apparatus is considerably simplified.
Advantageously, the total number of phase coding steps can be retained.
Although the resolution in the phase coding direction is reduced due to such a method, since the phase coding steps, i.e. the spacing of the gradient field pulses, is reduced and, further, the correctly analyzed field of view is thereby automatically enlarged, the original measuring time is preserved. This is particularly advantageous in measurements wherein a patient should hold his or her breath.
A new rectangular region within a field of view can be selected that should be free of over-convolutions and wherein the method is applied again.
A smaller region to be measured thus can be selected that can be measured free of over-convolution under all circumstances. In particular, the desired, increased resolution nonetheless can be nearly achieved. Due to the renewed application of the method, it is possible to prevent over-convolutions from occurring at least in the smaller, selected field of view.
A magnetic resonance tomography apparatus according to the invention has a control computer that is programmed to execute the above-described inventive method.
This magnetic resonance tomography installation exhibits the same advantages of the method.